Experimental investigation of inner bubble dynamics during water droplet evaporation from heated surfaces with different roughness and wettability levels

In this study, sol-gel method and picosecond laser system were used to create surfaces with various roughness and wettability levels. The ranges of contact angle and surface roughness were 0°–125° and 0.121–0.356 μm, respectively. The influence of surface roughness and wettability on bubble dynamics was experimentally investigated by observing droplet evaporation on heated surfaces. The results revealed that not only the surface temperature but also surface characteristics affected bubble behavior and resulted in different evaporation efficiencies. Droplets evaporated most rapidly on the laser-textured copper surface because of its hydrophobicity and enhanced surface roughness, which led to improved liquid–vapor heat transfer in active bubbles behavior. By contrast, silica-coated surfaces exhibited the lowest droplet evaporation efficiency. This evaporation efficiency was attributed to the hydrophilic wetting state and high bubble covering area, which hindered the heat transfer from the heated surface to the liquid droplet. The overall evaporation time on the laser-textured surface was approximately 77% shorter than that on other surfaces for the highest evaluated surface temperature. In addition, the experimental results of the overall evaporation time were compared with the results of the prediction model. With the increase in the temperature of the surface, the hydrophobic wetting state increased, which led to larger differences between the model and experimental results.